An ordinary ray of light is neither in a completely polarized state nor in a completely non-polarized state, and various states including the both states coexist. In order to obtain a specific polarized state from among those states, various types of polarizers have hitherto been studied.
As the above-described polarizers, there are, for example, a birefringent polarizer that utilizes double refraction of a crystal, a dichroic polarizer that utilizes optical dichroism of high polymer molecule, a reflective polarizer that utilizes a reflected light of an S-polarized light, and the like.
As an optical element having a function of the-above types of polarizer, there is known a polarizing beam splitter (PBS) in which, for example, a polarized light separation element transmitting therethrough a P-polarized light which is a first polarized light of an incident light entering at a predetermined angle, and reflecting an S-polarized light, which is a second polarized light, in a direction different from that of the transmission is constructed to be held between transparent substrates such as glass substrates.
Regarding the polarizing beam splitter (PBS), a lot of literatures concerned therewith are known, and, for example, an explanation is given of the polarizing beam splitter on pages 302 to 309 in “Light/Thin-Film Technique Manual, enlarged and revised edition” (published by The Optoronics Co., Ltd. in 1992).
In the above-described literature, there is shown a thin-film laminated type polarized light separation element constructed by alternately laminating a high refractive-index layer made of a high refractive-index material and a low refractive-index layer made of a low refractive-index material. That thin-film laminated type polarized light separation element utilizes optical single-axis anisotropy that occurs when two kinds of thin films (high refractive-index layer and low refractive-index layer) whose refractive indexes are different from each other and whose thickness are sufficiently small compared to the wavelength of light have been alternately laminated.
Regarding the thin-film laminated type polarized light separation element, when it is assumed that λ0 represents a reference wavelength of 550 nm of the incident light, L represents the low refractive-index layer having an optical film thickness of λ0/4 (the optical film thickness: the refractive index×the thickness), and H represents the high refractive-index layer having an optical film thickness of λ0/4, the above-described literature gives a designing guideline that is expressed as the following formulas (1), (2) or the like.(HL)m  (1)(0.5HL0.5H)m  (2)
where m is an arbitrary positive integer.
Also, the 0.5H represents a high refractive-index layer having an optical film thickness (λ0/8) that is 0.5 time as great as that of the high refractive-index layer H having a thickness of λ0/4.
In each of the above formulas (1) and (2), m is conventionally set to 4 or more.
The above-described thin-film laminated type polarized light beam separation element utilizes the difference in non-transmission range between the P-polarized light and S-polarized light when the light was made to obliquely enter.
Then, the polarized light beam separation element having the high refractive-index layer H and low refractive-index layer L repeatedly laminated is held by the transparent substrates to form a rectangular parallelepiped shape, thereby constructing a polarizing beam splitter.
A cross section of a conventional thin-film laminated prism polarizing beam splitter (PBS) is illustrated in FIG. 25.
In a polarizing beam splitter 50, a polarized light beam separation element 52 is constructed by alternately laminating the high refractive-index layer H and low refractive-index layer L repeatedly, and is held by transparent substrates 51 on both sides of the polarized light beam separation element 52.
The P-polarized light is transmitted through the polarized light beam separation element 52 as it is, while the S-polarized light is reflected by the polarized light beam separation element 52 and goes out obliquely downwards as shown in the figure.
Also, particularly, the beam splitter in which the angle of incidence upon the polarized light beam separation element is adjusted to be 45 degrees and the reflected S-polarized light goes out at an angle of 90 degrees with respect to the incidence direction is called “a rectangular prism PBS”.
FIG. 26 is a schematic construction view of a rectangular prism PBS 60.
A polarized light beam separation element 62 having a similar laminated structure to that of the polarized light beam separation element 52 illustrated in FIG. 25 is held by transparent substrates 61 from both sides thereof to construct the polarizing beam splitter (PBS) 60. Incident light enters the polarized light beam separation element 62 at an angle of incidence of 45 degrees with respect to the polarized light beam separation element 62, and an S-polarized light goes out downwards as shown in the figure at an angle of 90 degrees with respect to the incidence direction.
Hereupon, in the polarizing beam splitter using the above-described thin-film laminated type polarized light beam separation element, it is preferable that the angle of incidence of the incident light upon the polarized light beam separation element be set to a specific angle, mainly to an angle of 45 degrees (as shown in FIG. 26).
However, in that case, in order to satisfy desired characteristics, it is required to form a film having 30 layers or more as a multiple-layer film constituting the polarized light beam separation element, and as a result, the total film thickness becomes as thick as 4 μm or more.
Also, although the basic pattern in which the high refractive-index layer and low refractive-index layer are repeated is simple, practically the film thickness of each layer needs to be multiplied by a coefficient to be minutely adjusted.
Such multiple-layer film is difficult to manufacture and has a problem that it is impossible to be supplied cheaply.
Further, the manufacturing cost also increases.
Here, under the assumption that TiO2 is employed as the high refractive-index material for the high refractive-index layer H and SiO2 is employed as the low refractive-index material for the low refractive-index layer L, respectively, the calculated results of the polarized light beam separation characteristics of the polarized light beam separation film in which the number m that represents repetitions of time is set to m=4 to have a construction (HL)4, are shown in FIG. 24.
Then, as a standard, when it is assumed that the range satisfying the transmittance Tp of the P-polarized light being Tp≧90% and the transmittance Ts of the S-polarized light being Ts≦10% be regarded as a range in which the polarized light is sufficiently split (hereinafter called “the polarized light separable region”), as shown in FIG. 24, the bandwidth corresponding to the polarized light-separable region is from 380 nm to 540 nm, i.e. has a value of approximately 160 nm, and is not very wide.
Further, when a conventional optical glass of n=1.52 is, for example, employed as the transparent substrate with respect to the polarized light beam separation film having the above construction, the polarized light beam separation film exhibits optimum polarized light beam separation characteristics at an angle of incidence of about 58 degrees, and when the angle of incidence is 45 degrees, the film does not exhibit optimum polarized light beam separation characteristics.
Accordingly, it is understood that, when the polarized light beam separation film of the (HL)4 structure is constructed to be used, it becomes difficult to construct a high-performance PBS.
In order to solve the above-described problems, the present invention provides an inexpensive polarizing beam splitter having a function of splitting polarized light beam of the wide range with a simple film construction and a reduced number of laminated film layers, and a polarizer provided with the polarizing beam splitter to arrange natural light into a specific polarized state.